Heater wire



United States Patent HEATER WIRE Joseph F. Jacoby, Asheboro, N. C., Frank E. Becker,

Lexington, Ky., and Joseph Rosenberg, Erie, Pa., assignors to General Electric Company, a corporation of New York Application May 31, 1957, Serial No. 662,793

14 Claims. (Cl. 219-46) Thisinvention relates to wire used in electrically heated fabrics such as electric blankets and sheets; more particularly, the invention relates to heater wire composed of two or more conductors, one conductor being used as part of a heating circuit and the other conductor exercising an overheat control function.

Spooner and Greenhalgh Patent No. 2,581,212, which was assigned to the same assignee as the present application, disclosed an electric blanket having heater wire and a control circuit which prevented overheating of the blanket without the use of thermostats. The Spooner and Greenhalgh type of blanket wire has two conductors, one of which is the heating conductor, separated by a thermosensitive composition which is insulating at room temperature but which has such a high negative coefficient of thermal impedance that when the composition is heated to a temperature level representing an overheat condition, it becomes sufiiciently conducting for a disabling circuit to be energized through the two conductors. Related patents assigned to the same assignee as the present invention are Spooner-2,581,213; Saiford-2,745,- 943; and Price2,745,944. Another patent directed to such two-conductor heater wire is Voegler-2,725,447.

The above-mentioned patents are directed to compositions which are insulating at room temperature but which have such a high negative coefiicient of thermal impedance that they become sufficiently conductive at high temperature to enable a control function to be exercised by establishing an electric circuit therethrough. The Spooner and Greenhalgh patent discloses cellulose esters, vinyl halide resins, and polyamides as satisfactory compositions. The Saiford patent discloses a copolymer of dibasic carboxylic acid and polyhydric alcohol for this purpose. The Price patent discloses a butadieneacrylonitrile copolymer for this purpose. Filed of even date herewith is Rosenberg application Serial ,No. 662,792, which discloses and claims a copolymer of acrylonitrile and an acrylic acid ester as a satisfactory heater wire insulating composition having a high negative temperature-resistance coefficient.

In the above-mentioned patents, the control function was exercised through polymeric compositions which themselves had high negative temperature-resistance coefficients. It is an object of the present invention to provide heater wire in which the insulating composition has its negative temperature-resistance coeificient considerably enhanced by the presence of a small proportion of additive materials.

Other objects will appear from the following specification taken in conjunction with the annexed drawing wherein Fig. 1 is an enlarged elevation, partially in section, showing one form of heater wire embodying this invention; Fig. 2 is similar to Fig. 1 showing an alternative form of heater wire; and Fig. 3 is similar to Figs. 1 and 2 showing an additional embodiment of heater :vire.

2,846,560 Patented Aug. 5, 1958 Briefly stated, in accordance with one of its aspects, the invention is directed toward heater wire comprising a pair of spaced metallic conductors and a composition insulating at room temperature between the conductors, the composition having a high negative coeificient of thermal impedance provided by incorporating therein from 0.04% to 5% by weight of a surface active agent compatible with the composition and having a high coefficient of thermal impedance and low resistance.

We have discovered that the addition of certain surface active agents to insulating compositions greatly enhances the conductivity of such compositions at elevated temperatures without seriously affecting the insulating capabilities of such compositions at room temperature. The use of such additives not only results in improved properties in compositions previously used as heater wire insulation but widens the field of use to include compositions not previously considered satisfactory. While the reason for the improvement in properties is not known, the following explanation is given as a clarification of some of the factors involved. The correctness of the explanation is not vouched for, however.

The basis of electrical conduction through normally insulating compositions is the transport of a charged particle, either an electron or an ion, through the material. Very little electronic conduction takes place in insulating compositions; instead, such conduction as exists is primarily by ions. The number of mobile ions present in insulating compositions increases exponentially with temperature, and, therefore, the insulation resistance of materials such as nylon and polyvinyl chloride decreases exponentially with temperature. Many surface active agents are ionic materials and their addition to a normally insulating composition places foreign ions in the composition. These foreign ions, added to the ions already present in the composition, lower the insulation resistance of the composition at a given temperature. The more ions that are added, the lower the resistance becomes at a given temperature. While there is some tendency for insulating compositions with surface active agents as additives to have a lower resistance at all temperatures, since the effect of the additional ions is exponential the room temperature resistance of the compositions is not seriously reduced whereas the high temperature resistance is reduced sufficiently for a control function utilizing the conductivity of the composition to be readily exercised.

The requirements for a satisfactory surface active agent are:

(1) The material must contain sufiicient ions to lower the resistance of the insulating composition to the desired level;

(2) The amount of surface active agent required must not adversely affect the physical properties of the insulating composition;

(3) The surface active agent must be compatible with the insulation with which it is mixed;

(4) The agent must not be appreciably lost or altered under processing and usage conditions;

(5) The agent must be non-corrosive to the conductors in the event of intimate contact through migration.

The direct current resistance of a surface active agent is a measure of the ions it contains, low resistance indicating a large quantity of ions through which conduction is taking place. In addition to low resistance, a satisfactory surface active agent should have a coefficient of thermal impedance per degrees F. in excess of 45% as otherwise a disproportionate amount of surface active agent may have to be added in order to achieve the desired results.

One of the best surface active agents for use as an additive is stearyldimethylbenzyl ammonium chloride, sold commercially, as Triton K-60. The coefiicient of thermal impedance per 100 F. of this material is 82%, a figure considerably in excess of the 45% which is satisfactory for addition to many insulating compositions. The direct current resistance of this material is lower than is necessary for many insulating compositions. It is so satisfactory from the standpoint of resistance that we use it as a standard and have found that surface active agents having a direct current resistance at 80 F. of as much as three times the 80 F. resistance of stearyldimethylbenzyl ammonium chloride are satisfactory for addition to most insulating compositions. However, larger proportions of surface active agents other than stearyldimethylbenzyl ammonium chloride must be present in order to impart comparable properties to the insulating composition. Thus, as little as 0.04% by weight of stearyldimethylbenzyl ammonium chloride can bring about marked improvement in the properties desired in the insulating composition whereas as much as of other additive materials will have to be incorporated in the insulating composition. Typical insulating compo sitions which have their negative coefficients of thermal impedance increased by the addition of the surface active agents of this invention are oopolymers of acrylonitrile and acrylic acid esters; vinyl halides; polyamides (nylons); polyethylene; polypropylene; polytetrafluoroethylene and chlorotrifluoroethylene; isocyanates (polyurethanes); styrene-acrylonitrile copolymers; styrene-acrylonitrilebutadiene copolymers; polyesters such as the reaction product of ethylene glycol and ter-phthalic acid; the copolymer of dibasic carboxylic acid and polyhydric alcohol; butadiene-acrylonitrile copolymers; neoprene; butyl rubber; and natural rubber. The above list is intended to be representative only. In view of the rather wide dissimilarity in the various insulating compositions listed, it is obvious that the invention is of general application within the field of insulating compositions.

While stearyldimethylbenzyl ammonium chloride is the preferred surface active agent in accordance with this invention and polyvinyl chloride is the preferred insulating composition, other satisfactory surface active agents are cetyldimethylethyl ammonium bromide, sold under the trade name Ammonyx DME; alkyl imidazolinium,

sold under the trade name Quaternary-O; diisobutyl phenoxyethoxyethyldimethylbenzyl ammonium chloride, sold as Hyamine 1622; the sodium salt of the sulfate ester of an alkylphenoxy polyethanol, sold as Alipal Co-433; the ammonium salt of the sulfate ester of an alkylphenoxy polyethanol, sold as Alipal Co436; sodium hexadecyl sulfate, sold as Alropon; diethanol amine, sold as Alrosol B; alkamidopolyethylene glycol, sold as Aerotex Antistatic; penthadrate sodium metasilicate, sold as Crystamet; octadecyl-guanidine salts of octadecyl, cabonic acid and ethylene oxide, sold as Aerosol C-61; and N- octadecyl disodium sulfosuccinate, sold as Aerosol 18. The above list is intended to be representative rather than all-inclusive. Surface active agents generally which have an 80 F. direct current resistance no greater than three times that of stearyldimethylbenzyl ammonium chloride and a negative coefficient of thermal impedance per 100' F. in excess of 45% are satisfactory additives within the range of 0.04% to 5%.

Conventional methods of manufacturing may be applied to the production of the heater wire of this invention. For example, the surface active agents may be introduced to the insulating compositions by means such as a Banbury mixer and the resulting composition extruded over one or more of the heater wire conductors in the usual manner. Certain compositions are preferably worked on rolls before introduction to the extrusion machine.

Figs. 1-3 show typical constructions of combined flexible heater and thermosensitive elements embodying our invention. In Fig. 1, the structure 10 includes a preferably ribbon-like bare conductor 11 wound upon a flexible strand 12 of fiber glass, stranded cellulose acetate, or other suitable flexible insulation. Over the conductor and in intimate contact therewiththere is provided, as by extrusion, a layer or film 13 of the treated insulating composition, such as polyvinyl chloride with 0.5% by weight of stearyldimethylbenzyl ammonium chloride or other insulating composition previously described. Wound tightly on said layer 13 is a bare conductor 14, also advantageously ribbon-like, and then an outer insulation layer 15. The outer layer is selected from materials having good qualities of insulation, abrasion resistance and, for use in blankets and the like, ability to withstand laundering. We have found polyvinyl chloride suitable. The combination of the ribbon-like conductor 11 and flexible cord 12 is advantageous where the device is subject to frequent flexing, as in electric blankets or clothing. When used where there is no repeated flexing, a conventional solid or stranded conductor may be used and the core 12 eliminated.

In Fig. 2, the combined heater and thermosensitive element 10a has bare wires 16, 17, corresponding in function to the ribbon-like conductors 11 and 14 of Fig. 1, and tightly wound in parallel spaced relationship on the flexible insulating strand 18. The thermosensitive organic insulation 19, which, for example, may composed of a copolymer of acrylonitrile and acryhc acid ester to which has been added 5% of N-octadecyl disodium sulfosuccinate (Aerosol 18), is extruded thereon or otherwise applied thereto to substantially envelop and wires; and like the layer 13, the mass 19 serves to secure the wires in fixed spaced relationship. In the Fig. l embodiment, the active thermosensitive material of the layer 13 is represented by its radial thickness, whereas, in Fig. 2, the etfective resistance material eompnsesan inner layer extending between the adjacent turns of wires 16, 17.. We prefer to cover the paired wire 16, 17, with the common layer of thermosensitive material, rather than to have one wire coated therewith and an adjacent bare wire laid tightly thereagainst, because of the surer physical contact of each wire with the thermosensitive composition. In Fig. 2, the wall thickness of the layer 19 is not critical except insofar as it interposes insulation between the atmosphere and the inner thermosensitive layer between the spaced conductors, and it is entirely conceivable that the layer 19 may be made suitably thick to act as a protective cover for the structure, in which event the additional insulation layer 20 may be dispensed with. The active body of the thermosensitive material is sometimes referred to as the control layer.

Fig. 3 differs from Figs. 1 and 2 in that both the heater conductor 21 and control conductor 22 are first embedded in the thermosensitive composition 23, as by extrusion, and then are spirally wrapped around a mes senger strand 24. An outer protective coating 25 is then extruded thereover.

While the invention has been described with reference to certain specific embodiments, it is obvious that many variations coming within the spirit of the invention are possible. Therefore, it is intended that there no limitations in the invention other than those necessitated by the scope of the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electric heater wire comprising a pair of spaced metallic conductors, and a composition insulating at room temperature between said conductors, said composition having a high negative coefficient of thermal impedance'and having incorporated therein from 0.04% to 5% by weight of a surface active agent compatible with said composition, said surface active agent having 15 'a change in coefficient of thermal impedance per F. in excess of 45% and a direct current resistance at 80 F. no greater than three times that of stearyldimethylbenzyl ammonium chloride.

2. An electric heater wire as claimed in claim 1 wherein the insulating composition is polyvinyl chloride.

3. An electric heater wire as claimed in claim 1 wherein the surface active agent is stearyldimethylbenzyl ammonium chloride.

4. An electric heater wire comprising a non-conducting messenger strand, a pair of conductors spirally wrapped around said messenger strand, and an insulating composition having a high negative coefficient of thermal impedance provided by the presence therein of 0.04% to 5% by weight of a surface active agent compatible with said composition, said surface active agent having a change in coefficient of thermal impedance per 100 F. in excess of 45% and a direct current resistance at 80 F. no greater than three times that of stearyldimethylbenzyl ammonium chloride.

5. A heater wire as claimed in claim 4 wherein the conductors are in the form of ribbons.

6. A heater wire as claimed in claim 4 wherein the insulating composition is polyvinyl chloride.

7. A heater wire as claimed in claim 4 wherein the surface active agent is stearyldimethylbenzyl ammonium chloride.

8. An electric heater wire comprising a non-conducting center messenger strand, a first conductor spirally wrapped on said messenger strand, a layer of insulating composition around said first conductor, said insulating composition having a high negative coefficient of thermal impedance provided by the presence therein of 0.04% to 5% by weight of a surface active agent compatible with said composition, said surface active agent having a change in coefiicient of thermal impedance per 100 F. in excess of 45% and a direct current resistance at 80 F. no greater than three times the 80 F. direct current resistance of stearyldimethylbenzyl ammonium chloride, a second conductor spirally wrapped around said composition, and a protective outer covering around said second conductor.

9. A heater wire as claimed in claim 8 wherein the conductors are in the form of ribbons.

10. A heater wire as claimed in claim 8 wherein the insulation composition is polyvinyl chloride.

11. A heater wire as claimed in claim 8 wherein the surface active agent is stearyldimethylbenzyl ammonium chloride.

12. An electric heater wire comprising a non-conducting center messenger strand, a pair of conductors spirally co-wrapped in spaced relation around said messenger strand, and an outer coating of insulating composition around said conductors, said insulating composition having its negative coefficient of thermal impedance enhanced by the presence of 0.04% to 5% by weight of a surface active agent compatible with said composition, said surface active agent having a change in coefficient of thermal impedance per 100 F. in excess of and a direct current resistance at F. no greater than three times the 80 F. direct current resistance of stearyldimethylbenzyl ammonium chloride.

13. Heater wire as claimed in claim 12 wherein the insulation composition is polyvinyl chloride.

14. Heater wire as claimed in claim 12 wherein the surface active agent is stearyldimethylbenzyl ammonium chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,403,960 Stoops July 16, 1946 2,579,375 Eisen Dec. 18, 1951 2,581,212 Spooner et a1. Jan. 1, 1952 2,688,608 Weinstock Sept. 7, 1954 2,745,943 Safliord May 15, 1956 

1. AN ELECTRIC HEATER WIRE COMPRISING A PAIR OF SPACED METALLIC CONDUCTORS, AND A COMPOSITION INSULATING AT ROOM TEMPERATURE BETWEEN SAID CONDUCTORS, SAID COMPOSITION HAVING A HIGH NEGATIVE COEFFICIENT OF THERMAL IMPEDANCE AND HAVING INCORPORATED THEREIN FROM 0.04% TO 5% BY WEIGHT OF A SURFACE ACTIVE AGENT COMPATIBLE WITH SAID COMPOSITION, SAID SURFACE ACTIVE AGENT HAVING A CHANGE IN COEFFICIENT OF THERMAL IMPEDANCE PER 100* F. IN EXCESS OF 45% AND A DIRECT CURRENT RESISTANCE AT 80*F. NO GREATER THAN THREE TIMES THAT OF STEARYLDIMETHYLBENZYL AMMONIUM CHLORIDE. 